Figure 1-9
Simple TCP/IP Network: Three Routers with IP Addresses Grouped
First, note that Figure 1-9 shows some sample IP addresses. Each IP address has four num-
bers, separated by periods. In this case, Larry uses IP address 1.1.1.1, and Bob uses 2.2.2.2.
This style of number is called a dotted-decimal notation (DDN).
Figure 1-9 also shows three groups of addresses. In this example, all IP addresses that
begin with 1 must be on the upper left, as shown in shorthand in the figure as 1.__.__.__.
All addresses that begin with 2 must be on the right, as shown in shorthand as 2.__.__.__.
Finally, all IP addresses that begin with 3 must be at the bottom of the figure.
In addition, Figure 1-9 introduces icons that represent IP routers. Routers are networking
devices that connect the parts of the TCP/IP network together for the purpose of routing
(forwarding) IP packets to the correct destination. Routers do the equivalent of the work
done by each post office site: They receive IP packets on various physical interfaces, make
decisions based on the IP address included with the packet, and then physically forward the
packet out some other network interface.
IP Routing Basics
The TCP/IP network layer, using the IP protocol, provides a service of forwarding IP packets
from one device to another. Any device with an IP address can connect to the TCP/IP net-
work and send packets. This section shows a basic IP routing example for perspective.
NOTE
The term IP host refers to any device, regardless of size or power, that has an IP
address and connects to any TCP/IP network.
Figure 1-10 repeats the familiar case in which web server Larry wants to send part of a web
page to Bob, but now with details related to IP. On the lower left, note that server Larry has
the familiar application data, HTTP header, and TCP header ready to send. In addition, the
message now contains an IP header. The IP header includes a source IP address of Larry’s IP
address (1.1.1.1) and a destination IP address of Bob’s IP address (2.2.2.2).
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Chapter 1: Introduction to TCP/IP Networking 25
1
Addresses: 2._____
Larry
1.1.1.1
1
2
3
To 2._____
Send to R2
To 2._____
Send Locally
Always to
R1
Bob
2.2.2.2
IP
TCP
HTTP
Source
1.1.1.1
Destination 2.2.2.2
R1
R3
R2
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